Plain-English definitions of the pharmacokinetic terms used across DoseCurve: Cmax, Tmax, AUC, Vd, Ke, Cl, F, half-life, steady state and more.
A short reference for the pharmacokinetic vocabulary that shows up across DoseCurve and the pharmacology literature it draws on. Each term gets a plain-English definition, the standard mathematical symbol where applicable, and a one-line note on what it means in practice.
This is a reference page, not clinical guidance. Use it to read other Learn and Blog articles fluently; don't use it to make dosing decisions.
The time between administration and the drug reaching its peak plasma concentration. For an oil-depot intramuscular injection this is hours to days; for an IV push it is essentially zero. The DoseCurve model collapses the absorption phase into the moment of injection.
A compound produced by the body's metabolism of the parent drug that itself has pharmacological activity. Examples: testosterone → estradiol (via aromatase), testosterone → DHT (via 5α-reductase), codeine → morphine. The DoseCurve chart shows the parent compound only; metabolites are not modelled.
The total drug exposure over a defined time window, calculated as the integral of plasma concentration with respect to time. Units are typically ng·h/mL. AUC scales linearly with dose under first-order kinetics. The DoseCurve "average concentration" metric is closely related to AUC.
The fraction of an administered dose that reaches systemic circulation in active form. IV administration is defined as F = 1.0. Oral compounds are typically much lower due to first-pass hepatic metabolism. See bioavailability vs serum concentration.
A dose administered all at once, as opposed to as an infusion. The DoseCurve model treats every injection as an instantaneous bolus.
The volume of plasma cleared of drug per unit time, usually in L/h or mL/min. Clearance is the underlying determinant of how quickly a drug is eliminated. Half-life depends on the ratio of Vd to Cl.
The maximum plasma concentration achieved after a dose. For DoseCurve this is the "peak" metric, except DoseCurve reports it in mg-remaining-in-body rather than ng/mL.
A way of modelling drug distribution in the body as one or more well-mixed "compartments" with defined transfer rates between them. A one-compartment model treats the whole body as a single well-mixed volume; a two-compartment model adds a peripheral tissue compartment with its own equilibration kinetics. DoseCurve is a one-compartment model.
The early period after a dose during which the drug is moving from the central compartment (bloodstream) into peripheral tissues. On a two-compartment plot this is the early steep slope (alpha phase) that precedes the slower terminal slope (beta phase). The single-exponential DoseCurve chart does not show a separate distribution phase.
The fractional rate at which a drug is cleared per unit time, in 1/time units. Related to half-life by Ke = ln(2) / half_life. A larger Ke means faster elimination.
A chemical modification (attaching a fatty acid chain to a hormone) that increases lipophilicity and slows release from an injection-site depot. The longer the chain, the slower the release. See esters and release rates.
Elimination where the rate is proportional to the amount currently present. The mathematical signature is exponential decay. Most therapeutic doses of most drugs follow first-order kinetics. See first-order kinetics for injectables.
The time it takes for the amount of drug remaining in the body to halve. Under first-order kinetics, this is independent of starting amount. Steady state is typically reached after 4–5 half-lives of repeated dosing. See half-life explained.
Injection directly into muscle tissue. Faster absorption than subcutaneous for most formulations; standard route for vaccines and many depot esters. See SC vs IM.
An oil-based intramuscular injection that forms a slow-release reservoir at the injection site. The depot, not the parent molecule, is the rate-limiting step for absorption.
A larger initial dose used to reach a target plasma concentration faster than would be possible with the maintenance dose alone. Common with very long-half-life drugs where waiting 4–5 half-lives at the maintenance dose would be impractical. DoseCurve does not recommend loading doses; it will show their effect if you enter one.
The recurring dose that maintains a steady-state plasma concentration once the loading phase is complete. Calculated as dose = average concentration × clearance × interval / F.
Casual term for Cmax. In DoseCurve, the maximum value of the modelled mg-remaining curve in the visible window.
The concentration of drug in blood plasma, typically measured at a specified time after dosing. The clinically meaningful measurement for most lab-monitored drugs.
The fraction of drug bound to plasma proteins (mostly albumin, α1-acid glycoprotein, and for hormones, SHBG). Only the unbound (free) fraction is generally pharmacologically active. Changes in binding can shift the free fraction substantially without changing the total measured concentration.
A liver-produced glycoprotein that binds androgens and oestrogens with high affinity. SHBG levels modulate the bioactive (free) fraction of testosterone and other sex hormones, and vary substantially between individuals and over time.
The point in a repeated dosing schedule where the rate of drug entering equals the rate of elimination, so the average concentration stops climbing between cycles. Reached after approximately 4–5 half-lives of consistent dosing. See steady state.
Injection into the subcutaneous fat layer beneath the skin. Slower and more sustained absorption than intramuscular for most formulations. Standard route for insulin and most GLP-1 analogues.
The principle that the plasma concentration at any time after multiple doses equals the sum of the concentrations that would result from each dose given alone. Holds under first-order kinetics and is the mathematical basis of the DoseCurve chart.
A user-defined band on the chart representing the desired minimum and maximum levels. DoseCurve uses this for the "time in range" metric. It is not a clinical target — clinicians set those.
The time at which Cmax occurs after a dose. For instant-release IV this is essentially t=0; for oil-depot IM this can be hours to days post-injection.
Casual term for the minimum plasma concentration before the next dose. In DoseCurve, the minimum value of the modelled curve in the visible window after the first dose.
The apparent volume into which a drug appears to distribute, calculated as amount-in-body divided by plasma concentration. Not a physical volume; depends on drug properties and host physiology. See volume of distribution explained.
Elimination at a constant rate per unit time, independent of amount present. Occurs when elimination enzymes are saturated. Half-life is not a meaningful constant under zero-order kinetics. Examples: ethanol at typical recreational levels, phenytoin at high therapeutic concentrations.
Every term above is touched on in more depth in one of the Learn or Blog articles. Start with half-life explained and reading the curve if you are new.